Conventionally, a soldering method is widely employed for mounting electronic parts on a substrate. As methods for the embodiment of soldering, various methods are employed such as a method of forming metallic bumps by soldering as bonding electrodes disposed on electronic parts and a method of solder pre-coating for forming solder layers on the electrode surface of a substrate. Recently, from the viewpoint of environmental protection, solder containing almost no harmful lead, so-called lead-free solder, is generally employed in the above soldering.
Lead-free solder is greatly different in component and composition from conventional lead-type solder, and therefore, with respect to flux used in a soldering process, the one so far used in general cannot be used as it is. That is, conventional flux is insufficient in activation, resulting in inadequate removal of oxide film on the solder surface, and it is difficult to assure excellent solder wettability. To make up for the deficiency of such solder being poor in solder wettability, flux having such a composition that metal powder formed of metal having excellent solder wettability such as silver is mixed in the solder component is disclosed in Japanese Laid-Open Patent 2000-31210. By using such flux, it is possible in the reflow process to wet and spread molten solder along the surface of metal powder in the flux and to guide the molten solder to electrodes to be soldered. However, in the case of such flux, the following problem may arise depending upon the rate of metal powder contained.
Recently, since a non-washing process that omits washing for removing flux component after soldering (hereinafter called non-washing process) is mainly employed, flux component as residue remains sticking around soldered portions after the reflow process, and metal powder contained in the flux also remains around soldered portions.
Then, in case the amount of residual metal powder is considerable, there is a possibility of defective insulation caused due to migration. And if the amount of metal powder is reduced in order to prevent such defective insulation, it will result in lowering of the effect of guiding molten solder by metal powder in the reflow process and invite lowering of the soldering effect. Thus, a conventional soldering paste such as flux containing metal powder involves a problem such that it is difficult to satisfy requirements for obtaining both soldering effect and insulation effect.